This invention relates generally to compressor apparatus and, more particularly, to a method and apparatus for compressing a fluid in a centrifugal compressor with relatively high efficiencies and over a substantial operating range.
In a centrifugal compressor, it is desirable to convert the gas kinetic energy leaving the impeller to potential energy or static pressure. This is commonly accomplished by way of a diffuser which may be of either fixed or adjustable geometry. The fixed geometry diffuser may be of the vaneless type, or it may be of the fixed vane type. An adjustable geometry diffuser may be either of the vaned or vaneless type and take the form of a throttle ring as shown in U.S. Pat. No. 4,219,305 assigned to the assignee of the present invention, a movable wall as shown in U.S. Pat. No. 4,527,949 assigned to the assignee of the present invention, or include rotatable vanes as shown in U.S. Pat. No. 4,378,194 assigned to the assignee of the present invention. Each of these various types of diffusers have peculiar operating characteristics that tend to favor or discourage their use under particular operating conditions.
Centrifugal chillers used in air conditioning systems are normally required to operate continuously between full load and part load (e.g., 10 percent capacity) conditions. At this 10% flow condition, the air conditioning system still requires a relatively high pressure ratio (i.e., from 50-80% of the full load pressure ratio) from the compressor. This requirement puts an extreme demand on the stable operating range capability of the centrifugal compressor. Therefore, to prevent early compressor surge caused by impeller stall, centrifugal compressors are typically provided with a variable inlet geometry device (i.e. inlet guide vanes). Rotatable inlet guide vanes are able to reduce the flow incidence angle at the impeller under part load conditions, thus enabling stable compressor operation at much lower capacities.
In addition to the instability which may be introduced by the particular impeller and its inlet design, the diffuser may also be cause for instability under part load conditions. Of all types of diffusers, the vaneless type generally provides the broadest operating range since it can handle a wide variation of flow angles without triggering overall compressor surge. If variable geometry, such as is discussed hereinabove, is added to such a vaneless diffuser, further stability can be obtained, but such features add substantially to the complexity and costs of a system.
Typically associated with the broader operating range of a vaneless diffuser is substantially lower efficiency levels because of the modest pressure recovery in the diffuser. The vaned diffuser, on the other hand, allows higher efficiencies but generally demonstrates a substantially smaller stable operating range. To increase this operating range, some type of variable diffuser geometry may be added to the vaned diffuser to prevent surge when operating under off-design conditions so as to thereby obtain relatively high efficiency over a broad operating range. But again, such a structure is relatively expensive.
One type of fixed geometry diffuser that has demonstrated an exceptionally higher efficiency level is that of the fixed vane or channel diffuser, which may take the form of a vane island or wedge diffuser as shown in U.S. Pat. No. 4,368,005, or a so-called pipe diffuser design as shown in U.S. Pat. No. 3,333,762. The latter was developed for efficiency improvement under transonic flow conditions occurring in high pressure ratio gas turbine compressors. Like other vaned diffuser compressors as discussed hereinabove, higher efficiencies are obtained, but they normally introduce an associated narrow stable operating range, which for the gas turbine compressor is not of concern, but when considered for centrifugal chiller application is of significant concern as discussed hereinabove.
In one instance as shown in U.S. Pat. No. 4,302,150, a pipe diffuser was used, supposedly to obtain higher efficiencies, with the associated narrow operating range being broadened by the introduction of a so-called vaneless diffuser space between the impeller outer periphery and the entrance to the diffuser. However, the increased stability of such a design is minimal and only occurs under full load operating conditions (i.e., no inlet guide vanes). Further, the larger vaneless diffuser space reduces the compressor lift capability under part load conditions. Moreover, the introduction of a relatively large vaneless space tends to move the peak efficiency closer to the surge point, an operating condition that cannot be tolerated for safe compressor operation.
In addition to the design considerations for the diffuser as discussed hereinabove, the impeller design features can also be chosen so as to generally optimize efficiency and operating range. While it is generally understood that impeller efficiency peaks when its blade exit angle .beta..sub.2 approaches 45 degrees (as measured from the tangent direction), there is also a general understanding that, to a point, the operating range of a centrifugal compressor increases as the impeller blade exit angle .beta..sub.2 decreases. For a given ratio between the impeller inlet relative velocity and the impeller exit relative velocity, reducing the impeller blade exit angle .beta..sub.2 (i.e., increasing the backsweep) will reduce the absolute flow exit angle .beta..sub.2 leaving the impeller. If this angle .alpha..sub.2 decreases too far, however, the radial pressure gradients near the impeller periphery tend to cause flow separation, and the operating range thus becomes narrower. Therefore, in centrifugal refrigeration impeller practice, the impeller absolute flow exit .alpha..sub.2 angle is normally chosen to be within the range of 20 and 40 degrees. Further, heretofore, it was generally understood that to reduce the impeller flow exit angle .alpha..sub.2 below 20 degrees would inherently lead to flow separation and a narrowed operating range. The use of impellers with such flow exit angles have thus been avoided.
It is, therefore, an object of the present invention to provide an improved centrifugal compressor method and apparatus.
Another object of the present invention is the provision for a centrifugal compressor which demonstrates high efficiency and a broad stable operating range.
Yet another object of the present invention is the provision in a centrifugal compressor for obtaining higher efficiencies without any substantial loss in operating range.
Still another object of the present invention is the provision in a centrifugal compressor for a diffuser apparatus which is effective in use and economical to manufacture and operate.
Still another object of the present invention is the provision for a centrifugal compressor which is economical to manufacture and effective in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.